The activity of tryptophan hydroxylase, the rate-limiting enzyme in the synthesis of the neurotransmitter serotonin, was determined in extracts of various rat brain structures after treatments which increase or decrease the availability of serotonin in the synpatic cleft. These treatments included the clinically effective antidepressant agents phenelzine, tranylcypromine, clorgyline, pargyline and lithium. Tryptophan hydroxylase is significantly increased in the mesencephalic tegmentum after three or six weeks of phenelzine treatment. Clorgyline, pargyline and lithium did not alter tryptophan hydroxylase activity.